A typical X-ray micro-computed tomography (“CT”) system for industrial applications consists of an X-ray source, a manipulator/rotator for positioning and rotating the object to be scanned, and an X-ray detector (camera). A good CT scan requires the accurate determination of the central ray. The central ray is sometimes called the iso-channel and is the virtual projection of the center-of-rotation on the detector.
Most fan-beam micro-CT inspection systems are also equipped with two-dimensional inspection as their basic capability. These systems frequently require that the manipulator move from place-to-place; and that the rotator be mounted on, and removable from, the manipulator. The consequence of this flexibility is the prerequisite determination of the central ray each time the manipulator or the rotator is moved. Even with the same manipulator coordinates, generally the system will give two central ray positions that may be sufficiently different to be unacceptable.
A common solution to this problem is to use a wire phantom to calibrate the central ray position before each CT scan. To do so, the wire phantom is placed on the rotational axis and is rotated for 360 degrees in predetermined angular steps. The projections of the wire phantom at all angles are then recorded and used for the determination of the central ray. The wire phantom is usually quite small so that it can be treated as a point for all angles. With fan-beam geometry, due to the small deviation of the wire phantom to the axis of the rotation, the central ray is simply determined as being center of the sinogram of the wire phantom.
The use of a wire phantom to determine the central ray position creates many problems including, but not limited to: reducing the speed of the CT process; introducing errors when changing the object for the wire phantom due to different weight, different fixing status, and so forth, thereby affecting the final CT image quality; and CT scans cannot be automated. Large errors result when large magnification is needed, and the object has to be placed close to the source. In this case, the manipulator is required to move away from the source so that there is space for changing the object for the wire phantom and vise versa.